1. Field of the Invention
The present invention relates, in general, to image forming apparatus. In particular the invention relates to an electrophotographic image forming apparatus wherein at least two-color images are formed on a photosensitive drum acting as an image carrier when the drum makes a single turn.
2. Description of the Prior Art
In image forming apparatus in which a plurality of color images are formed on an image carrier byusing a plurality of color developing agents digitized image data are converted into light image data by a light-emitting element or a photo-switching element. The light image data are formed on the image carrier as an electrostatic latent image. A laser diode and a light-emitting diode are used as the light-emitting element. A liquid crystal and an element using the Faraday effect are used as the photo-switching element. In this type of image forming apparatus, each color of a document image is designated by an operator. The latent image corresponding to the document image is developed by each color developing agent in accordance with the designated color data of the document image.
FIG. 1 shows one example of the image forming apparatus described above in FIG. 1, a photosensitive drum 21 rotates clockwise. The surface of photosensitive drum 21 is coharged at a prescribed surface voltage level by a first charger 23. A first electrostatic latent image is formed on drum 21 by a laser beam 25, which scans the surface of drum 21 in response to image data from a host system. The first latent image on drum 21 is developed by a first developer 27 in which a first color developing agent 27a is stored. Following the first color developing Operation, the surface of drum 21 carrying the developed first color image is recharged to a prescribed surface voltage level by a second charger 29. A second electrostatic latent image then is formed on drum 21 by a laser beam 31. The second latent image on drum 21 is developed by a second developer 33, in which a second color developing agent 33a, different from the first color developing agent 27a, is stored. At this time, the first color developing agent and the second color developing agent are present on the same surface of drum 21. The first color developing agent and the second color developing agent are transferred onto a sheet 35 in a visible image by a transfer charger 37 as sheet 35 is conveyed in the direction indicated by arrow A. Before the transfer process described above, the charged polarity and the charged quantity between the first developing agent and the second developing agent on drum 21 are equalized by a pre-transfer charger (not shown), if necessary. After the transfer process described above, the residual first and second color developing agents are removed by a cleaner 39, and the latent images are erased by a discharge lamp 41. Thus, the printing process is completed, and the image forming apparatus stands by for the next print operation.
A similar construction of two color image laser printer is disclosed in Nikkei Electronics, pages 123 to 132, issued on Oct. 6, 1986.
It is believed that the above-described type of image forming apparatus has not been used previously for making single color prints using one of the plurality of color developing agents. One reason may be that the second or subsequent color developing agents can become contaminated when only a single color image is developed, but multiple chargings and biasing controls are necessary, as discoverd by the present inventors.